Metal foil-making process

ABSTRACT

A PROCESS FOR MAKING METALLIC FOIL CONSISTING OF A SUBSTATIALLY HARD AND NONDEFORMABLE METAL OR METAL ALLOY WHICH COMPRISES REDUCING THE METAL OR METAL ALLOY TO A POWDER AND THEREAFTER BONDING THE POWDER BY MEANS OF A VOLATILE ORGANIC BINDER INTO A THIN SHEET WHICH THEREAFTER IS SINTERED IN VACUUM AT AN ELEVATED TEMPERATURE WHILE DISPOSED BETWEEN TWO SUBSTANTIALLY FLAT REFRACTORY SURFACES, FORMING THEREBY AN INTEGRAL METALLIC FOIL WHICH IS SUBSTANTIALLY DEVOID OF ANY RESIDUAL BINDER CONSTITUENT.

United States Patent 3,809,553 METAL FOIL-MAKING PROCESS Robert L. Peaslee, 217 Linden, Royal Oak, Mich. 48073 No Drawing. Filed Dec. 26, 1972, Ser. No. 318,096 Int. Cl. B22f 3/14, 3/18, 3/22 U.S. Cl. 75213 Claims ABSTRACT OF THE DISCLOSURE A process for making metallic foil consisting of a substantially hard and nondeformable metal or metal alloy which comprises reducing the metal or metal alloy to a powder and thereafter bonding the powder by means of a volatile organic binder into a thin sheet which thereafter is sintered in vacuum at an elevated temperature while disposed between two substantially flat refractory surfaces, forming thereby an integral metallic foil which is substantially devoid of any residual binder constituent.

BACKGROUND OF THE INVENTION The production of metallic foils of metals and metal alloys which are relatively soft and ductile is conventionally achieved by subjecting east slabs or ingots of the material to a series of roll reduction passes which may include one or more intervening annealing treatments to avoid excessive work hardening of the rolled billet. A variety of problems have been encountered when applying such conventional foil-making processes to metals and metal alloys which are of comparatively greater hardness, necessitating the use of elaborate rolling equipment and sophisticated annealing treatments in order to produce metallic foils of less than about 0.010 inch thick.

There are a large number of metal alloys which, because of their great hardness and lack of ductility, have up until now been found to be incapable of being produced in the form of metallic foils employing prior art foil-making processes. Typical of such hard and nonductile metals are the so-called nickel and/or cobalt-base brazing filler metals and hard surfacing alloys which heretofore have not been commercially available in the form of metallic foils for advantageous use in certain brazing and hard surfacing processes. Because of the commercial impracticality of fabricating such brazing filler metals and hard surfacing alloys as metallic foils, it has been suggested in the past to comminute the metals into a fine-sized powder which is admixed with an organic binder or cement and cast into thin sheets and dried. The binder used is one which is intended to volatilize during the elevated temperatures encountered in the brazing furnace.

While such composite sheets of organic bonded metallic powder have provided a satisfactory expedient in the past, the presence of the organic binder in amounts as high as 25 volume percent of the sheet necessitates furnace conditions which are conducive to achieving a substantially complete volatilization of the binder to avoid the presence of any deleterious residues in the brazing filler metal. The use of the brazing filler metal in particulated form at a density of about 60% of theoretical density does not provide sufficient filler metal in some instances to adequately fill the gap at the joint.

The process of the present invention provides a commercially practical method for producing foils of hard nonworkable metals, metal alloys, intermetallic compounds and/or nonmetallic compounds, as well as mixtures thereof, which are of a controlled and uniform thickness, homogeneous in composition and devoid of any extraneous binder materials, providing therewith increased versatility in use and optimum performance of the foil material.

Patented May 7,, 1974 The benefits and advantages of the present invention are achieved by a process in which materials, and principally metal alloys of a hardness greater than about 35 as measured on the Rockwell C scale, are comminuted into a powder of a maximum particle size of less than about 250 microns, whereafter the resultant powder is admixed with a volatile organic binder in an amount of up to about 25% by volume of binder on a solids basis, and thereafter cast in the form of a thin sheet of a thickness of up to about 0.015 inch. The resultant cast composite sheet is thereafter placed between two substantially flat temperature-resistant platens and placed within a furnace, preferably provided with a vacuum atmosphere, and heated to a temperature at which a volatilization and substantially complete removal of the organic binder occurs, accompanied by a concurrent incipient melting and diffusion bonding of the particles to each other, forming a sintered foil of a thickness usually less than about 0.012 inch. The resultant sintered foil can be employed in that form, but preferably is subjected to a rolling or coining operation for the purposes of sizing and may further be trimmed or cut to standard sizes to facilitate its handling during shipment and at the time of its ultimate use.

It will be understood that while the process is hereinafter described in connection with the formation of metallic foils composed of metal alloys suitable for brazing and hard surfacing, the process is equally applicable for fabricating foils of other metals, metal alloys, intermetallic compounds and nonmetallic compounds, as well as mixtures thereof, which similarly cannot practically be formed into foils using conventional processing techniques. Addi-. tional advantages and the further applicability of the process comprising the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the typical examples set forth.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The process comprising the present invention is applicable for the fabrication of foils from metals or metal alloys which are of a hardness generally of at least about 35 Rockwell C (Re) and which are further characterized in that powders of such metals cannot be compacted in the absence of binding agents to provide a mass or billet with sufiicient green strength to enable sintering thereof. The poor green strength exhibited by compacts of such powders, in spite of the use of comparatively high pressures, is to a large extent attributable to the absence of any appreciable embedability and/or mechanical interlocking between the individual powder particles due to their hardness. The problem is further aggravated by increases in hardness of the powder particles as a result of their work hardening during the compacting process Such metal powders can be further characterized as those which generally exhibit poor atomic bonding or solid phase welding between particles at the compacting pressures and temperatures employed. It is important, however, that the powdered materials employed in the practice of the present process possess the capacity of being sintered or diffusion bonded when heated to elevated temperatures, forming therewith a three-dimensional matrix in the form of a foil having sufficient strength to enable a further handling thereof.

As previously indicated, the process as herein described is particularly suitable for producing foils of nickel and/ or cobalt base and other stainless steel alloys which may or may not possess self-fluxing properties and are of 'a type suitable for use as brazing filler metals and as hard' surfacing metals. Several metal alloy compositions suitable for use as brazing filler metals are provided in Table 1.

TABLE 1 Nominal composition, percent by weight solution in a volatile solvent. The mixing of the binder and powder is continued until a substantially homo- Melting point, F

Si W Fe Cu O Mn Co Ni Solidus Liquidus 1. 780 l, 900 l, 780 1, 830 l, 975 2, 075 l. 800 1, 850 l, 800 1, 900 l. 810 1, 935 2, 025 2, 100

1 Maximum.

The specific metal or metal alloy, as well as mixtures of pure metals, metal alloys, intermetallic compounds and nonmetallic compounds, are first reduced to a finelydivided powder form employing any one of the techniques well known in the art. While chemical reactions and decompositions, as well as electrolytic deposition of materials, can be employed in many instances for producing powders of selected materials, the more common and commercially acceptable techniques include a mechanical processing or comminution of solid masses of such materials and preferably an atomization of molten masses of such materials or alloys thereof into a plurality of fine-size droplets.

While the specific source of the powdered constituents or their method of manufacture is not critical, different techniques produce powder particles of different configurations and will, therefore, influence, to some extent, the manner by which the powder is blended, the type and quantity of binder employed and the conditions under which the material is sintered in order to achieve optimum results. For example, the manufacture of metal alloy powders employing atomization of molten masses of such alloys corresponding to the compositions as typically set forth in Table 1 results in powder particles which are generally of a spherical configuration due to the high surface tension of such metals. Mechanical comminution techniques, on the other hand, generally produce metal powders in which the particles are of an irregular shape or flake configuration. Regardless of the particular method employed for manufacturing the powder, the particles thereof are selected so as to be of a maximum size of less than about 250 microns, and preferably of a size ranging from about 30 down to about 5 microns. In accordance with a preferred practice, the particles are selected so as to be distributed over the permissible size range in order to provide for optimum packing.

When metallic foils comprised of mixtures of two or more particulated materials are to be produced, appropriate amounts of the individual powdered materials are preliminarily mixed to form a blend of substantially uniform composition throughout, whereafter a solution comprised of a volatile solvent containing a resin binder is added ranging in an amount from as little as about /2% by volume up to about 25% by volume (measured on a solids basis), and preferably from about 6% to about by volume. The binder solution can usually contain from as low as about 5% to as high as about 50% resin solids by weight with the balance volatile solvent. Since the binder employed is selected among those which substantially completely volatilize and/or decompose without residue during the sintering of the powder layer, it is usually preferred to employ only that quantity of binder necessary to achieve satisfactory green strength, thereby maximizing the density of the resultant sintered foil. Binding agents of the types suitable for use in accordance with the practice of the present invention include acrylic resins, acrylic acids, polyvinyl alcohol, etc.

The appropriate quantity of binding agent or mixture of binding agents is added to the metal powder or powder blend by utilizing an appropriate mixer, such as a doublecone mixer, and wherein the binding agent is introduced in the form of a finely particulated powder, a liquid or a geneous blend is attained, whereafter the mixture is cast, doctored, rolled or otherwise spread in the form of a thin layer of a thickness less than about 0.020 inch, and more usually of a thickness ranging from about 0.015 to as low as about 0.003 inch. The resultant layer is thereafter dried, cooled or otherwise cured, depending upon the particular type of binder employed, whereafter the resultant sheet is subjected to sintering in accordance with the conditions hereinafter set forth.

Since the sintering step is carried out such that the sheet is disposed between two heat-resistant slabs or platens, the powder/binder mixture can be directly cast, doctored or otherwise applied on the surface of one such slab, eliminating the need for handling the sheet in an unsupported condition. Alternatively, the powder/binder mixture can be cast or doctored on a flat surface having a release coating thereon and the sheet, after drying, is transferred to the platens.

The sintering of the powder into an integral matrix and the volatilization of the binding agent is preferably performed in a furnace having an evacuated atmosphere and wherein the shwt is subjected to a substantially uniform temperature over its entire area. While atmospheres comprised of inert gases, such as argon and helium, or nonoxidizing atmospheres such as pure dry hydrogen, can be employed during the sintering step, it is usually preferred to perform the sintering in vacuums of less than about 50 millitorrs (microns of mercury absolute) and preferably vacuums of less than about one millitorr. It is also preferred to preliminarily flush the interior of the furnace with an inert gas in order to purge the air therefrom so that any residual gases remaining after the vacuum is drawn will not contain any reactive oxygen.

The sintering of the powder sheet is performed at a temperature at or slightly above the incipient melting point of the metal or metal alloys and at which temperature a relatively rapid bonding by atomic diffusion and neck growth between adjoining particles occurs. The term sintering, as herein employed, is intended to encompass the usual solid state diffusion process in which no liquid phase is present, as well as so-called liquid phase sintering in which the sintering temperature is at or. slightly above the solidus such that one or more compo nents of the powder is present as a liquid during all or a part of the sintering process. The proper sintering tem perature for alloys comprised of a plurality of constituents which have a melting point extending over an appreciable range is selected at or about the solidus temperature of the alloy. A comparison of typical solidus and liquidus temperatures of metal alloys is provided in Table l.

A heating of thin layers of powder substantially above the solidus temperature during the sintering operation is undesirable because of the formation of excessive amounts of liquid which coagulates, resulting in a sintered foil which is either discontinuous or one which is of nonuniform thickness and/or density. A heating of such metal powders to temperatures at or above the liquidus in which all of the powder is in liquid form results in the formation of pools or puddles of the metal in substantial thickness in spite of the pressure exerted by the refractory platens. It is for the foregoing reason that the sintering operation must be carried out at a temperature below or at about the solidus temperature to avoid any appreciable formation of a liquid phase.

The sintering step is carried out for a period of time sufficient to form a diffusion bond or neck between particles at their points of contact of sufiicient strength such that the sintered foil can be readily handled in normal use without encountering breakage or disintegration. The sintering operation is accompanied by a reduction in the thickness of the metallic layer or sheet in amounts up to about 50%, which will vary depending on the size and configuration of the powder particles, the quantity of organic binder employed and the sintering conditions used. Appropriate compensations are accordingly made in the original powdered sheet in order that the resultant sintered foil will range in thickness of from about 0.010 to about 0.002 inch.

At the conclusion of the sintering step, the sintered foil is cooled and thereafter is preferably subjected to further compaction such as by rolling or coining to more accurately size the foil and increase the uniformity in the thickness thereof across its entire area. The resultant sintered foil is of a density usually greater than about 95% to 100% theoretical density, and is of a strength suflicient to enable trimming thereof into sheets of appropriate size or strips which can be coiled to facilitate its shipment and storage prior to use.

In order to further illustrate the process comprising the present invention, the following example is provided. It will be understood that the example is provided for illustrative purposes and is not intended to be limiting of the scope of the invention as herein described and as defined in the subjoined claims.

EXAMPLE A quantity of a metal alloy powder having a nominal composition corresponding to Alloy No. 135 of Table 1, an average particle size ranging from about 5 to about 44 microns and a substantially spherical particle shape is admixed with 50% by volume of a binding agent solution consisting of 18% by weight solids of an acrylic resin in an organic solvent. A substantially uniform blend of the metal alloy powder and the binding agent solution was achieved in a rotary mixer.

The resultant blend, which was of a slurry-like consistency, is doctored in the form of a uniform layer of about 0.010 inch thick on the flat surface of a siliconecoated release paper and allowed to dry. The dried sheet is thereafter transferred to one surface of a heat-resistant ceramic platen. A second ceramic platen is placed in overlying relationship on the dried sheet and the assembly is placed in a furnace of a type suitable for vacuum brazing, which is preliminarily purged with argon, whereafter a vacuum of less than about one millitorr is drawn. The furnace is slowly heated to a sintering temperature of about 1890 F. and held at that temperature for a period of 1% hours, whereafter the sintered strip is cooled and subjected to a light rolling operation, if desired, to effect a sizing in its thickness. The resultant sized foil is of a thickness of about 0.004 inch, a density of greater than about 98% of theoretical density and sutficient strength to enable its trimming and deformation as may be required during its use as a brazing filler metal.

While it will be apparent that the invention herein described is well calculated to achieve the benefits and advantages hereinbefore set forth, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.

What is claimed is:

1. A process for making a metallic foil comprised of metallic particles of a hardness greater than about 35 Rc which comprises the steps of reducing the metallic material to a powder having a particle size of less than about 250 microns, admixing said powder with an organic binder in an amount of up to about 25 volume percent of said binder on a solids basis forming a cohesive powder mixture, forming said cohesive powder mixture into a uniform layer of a thickness less than about 0.020 inch, positioning said layer between two substantially flat heat-resistant platens disposed in spaced substantially parallel relationship and heating said layer in a nonoxidizing atmosphere to a temperature sufficient to effect a sintering and diffusion bonding of the powder particles into an integral three-dimensional matrix accompanied by an evolution and substantially complete removal of said binding agent, and thereafter cooling the resultant sintered foil.

2. The process as defined in claim 1, in which the step of reducing the metallic material to a powder is carried out to produce a powder having a particle size ranging from about 5 microns to about 30 microns.

3. The process as defined in claim 1, in which said cohesive powder mixture is formed into said uniform layer having a thickness of from about 0.003 to about 0.015 inch.

4. The process as defined in claim 1, in which said organic binder is present in an amount of from about 6% to about 15% by volume on a solids basis.

5. The process as defined in claim 5, in which the sintered foil is of a thickness of from 0.002 to about 0.010 inch.

6. The process as defined in claim 1, wherein said nonoxidizing atmosphere comprises a vacuum of less than 50 millitorrs.

7. The process as defined in claim 1, wherein said nonoxidizing atmosphere comprises a vacuum of less than one millitorr.

8. The process as defined in claim 1, wherein said nonoxidizing atmosphere comprises a substantially dry inert gas selected from the group consisting of helium, argon and mixtures thereof.

9. The process as defined in claim 1, wherein said nonoxidizing atmosphere comprises a substantially dry pure hydrogen atmosphere.

10. The process as defined in claim 1, in which said layer is heated to a sintering temperature at or slightly above the solidus of said metallic material producing some liquid phase during the sintering process.

References Cited UNITED STATES PATENTS 3,658,517 4/1972 Davies et al 214 X 3,627,519 12/1971 Baker 75225 X 3,115,698 12/1963 St. Pierre 75211 X 2,698,990 1/1955 Conant et a1. 75211 X 2,573,951 11/ 1951 Brennan 75226 X OTHER REFERENCES Thellmann, E. L.: Slip Casting-A Versitile P/M Process, in Precission Metal, 29(5):41-43, 1969.

CARL D. QUARFORTH, Primary Examiner R. E. SCHAFER, Assistant Examiner US. Cl. X.R.

W A 'PATilNT flmag CERTEFi-QATE 0F (ZOERREQT Petent No. 3,809, 553 I :D t d May 7, 1974 Ifiventor(s); ert'L. Peaslee It is "certified-that error appears in the above-identified patent and that said Letters Patent are hereby cerr'ected as Shawn below:

Column 1 irisert *=-Assignee: Wa11 Coimohoy Corporation,

Detroit Michigan v Celumn 6 Claim 5, "c1airn 5" should be --c 1aim 1-.

Signed and sealed this 22nd day of October 1974.

(SEAL) Attest: I

7 McCDY M. GIBSON JR. I c. MARSHALL DANN Attesting Officer Commissioner of Patents I'Y-"ORNVI 90-1050 uo esi v r v uscwwoc mumps;

U.S. GOV ERNHENT PRINTING OFFICE I909 O-JGi-SJA UNiT o STATES PATENT oFFicE CERTIFICATE 013 I CORRECTION Patent No. 3,809,553 i Da May 7, 1974 Ioventofls) ert L. Peaslee It is certified that error appears in the above-identified patent and phat said Letter-s Patent are hereby corrected as shown below:

Column 1 irisrt -Assigne:,Wa11 Coirnorioy Corporation,

Detroit, Michigan Column 6 C1aim 5, "claim 5" should be --o1aim 1-'-.

Signed and sealed this 22nd day of October 1974.

(SEAL) AtteSt: v

MCCOY M. GIBSON JR. i 3. MARSHALL DANN Attesting Qfficer Commissioner of Patents FORM Po-1o5o (was) USCOMWDC 5037M,

U.S. GOVFINHINT PIINTXNG OFFICE: 19'9 O-JSi-Sl 

